1. Field of the Invention
This invention relates to bulk semiconductor devices and, more particularly, to opto-electronic devices grown on bulk crystals.
2. Art Background
Photodetectors having wavelength responses tailored to specific frequencies in the infrared are becoming important for applications such as optical communications. Bandgaps of the absorbing semiconductor material in this photodetector must have an energy bandgap sufficiently small to provide a response in this low energy region of the spectrum. Ternary and quaternary semiconductors are being investigated for this purpose since the composition of these compounds can be varied to produce a wide range of bandgaps. A combination of two binary semiconductors, at least in theory, can be used to produce bandgaps between those of the constituent binary semiconductors. The III-V compounds, in this regard, are attractive since they have bandgaps in the desired infrared region for optical communications. For example, binary materials such as InP or InAs have been combined to make epitaxial ternary layers of InP.sub.y As.sub.1-y, which, depending on the value of y, have a bandgap between 0.36 eV and 1.34 eV at room temperature.
For many photovoltaic applications, including photodetection, it is desirable to use single crystal devices, i.e., devices which are built on a single crystal semiconductor material which has a low defect density, (typically less than 10.sup.6 cm.sup.-2) and which is of high purity, i.e., typically having less than 10.sup.16 carriers/cm.sup.3 in nominally undoped crystals. Such single crystal devices, including those based on III-V compounds, generally have much better quantum efficiencies than their polycrystalline counterparts. These higher efficiencies are desirable for applications where a small area device is used to detect low intensity radiation.
Two types of single crystal devices are the most prevalent. The first is-built upon an epitaxial layer (typically 1-10 .mu.m thick) of semiconductor material deposited on a single crystal substrate of different composition. In this configuration, the substrate must be matched to the underlying epitaxial layer so that good electrical properties, approaching those of bulk crystalline material, are achieved in the epitaxial layer. This, at the very least, requires matching the lattice parameters of the device semiconductor to that of the substrate.
In ternary semiconductors, when the composition is adjusted to give the appropriate lattice constant for a given substrate, there is no further possibility of adjusting composition to form a desired bandgap. Therefore, an epitaxial single crystal device, grown on bulk single crystal substrates of a binary or elemental semiconductor, generally cannot be tailored to operate at a desired wavelength. In a quaternary alloy, which has one greater degree of compositional freedom than the ternary, lattice matching and bandgap tailoring is possible for an epitaxial device using certain binary substrates. (See J. Electron. Mater., 6, 253 (1977).) However, in practice, controlling the composition to obtain precise lattice matching is difficult. Additionally, other parameters such as dopant levels must also be controlled during epitaxy of a quaternary compound. Because of these extensive control complications, it is generally difficult to obtain reproducible results for quaternary epitaxial layers.
In a second type of single crystal structure, devices such as Schottky barriers, heterodiodes or homodiodes, built on bulk crystals (crystals having dimensions greater than 5 mm) of a semiconductor or wafers cut from such crystals have advantages over their corresponding epitaxial entity. Since the device is built directly on the bulk crystal of an active semiconductor, i.e., a semiconductor used to form the rectifying interface, lattice matching is not a problem. This extra degree of freedom results in the possibility of producing a chosen bandgap in ternary devices and relieves the control problems associated with quaternary devices. Other fabrication problems are also markedly reduced. For example, once the bulk crystal is formed, devices with reproducible physical dimensions can be produced merely by slicing wafers from the bulk crystal body.
Although bulk crystals have inherent advantages, their growth is not a trivial problem. Bulk crystals with high defect densities which are not satisfactory for device utilization are often produced. The formation of high quality single crystal materials, i.e., materials having less than 10.sup.6 cm.sup.-2 defects as measured by methods such as etchpit and x-ray techniques (Thin Solid Films, 31, 185 (1976); ibid, 31, 253 (1976); J. Appl. Phys., 36, 2855 (1965); J. Electrochem. Soc. 107, 433 (1960)), is often not achievable. For example, ternary III-V systems typically have large gaps between their liquidus and solidus over their entire compositional range. This property generally produces both radial and axial compositional variations in crystals grown from the melt. Both such gradients lead to poor reproducibility and often to inoperative devices. The radial variations are particularly unacceptable since wafers are typically cut perpendicular to the growth direction of the crystal. This method of cutting, in radially defective crystals, yields unacceptable gradients across the surface of the semiconductor material upon which the device is built. For quaternary compounds, the solidus-liquidus gap is usually even more unfavorable. Thus, the probability of making a single crystal bulk quaternary device with reproducible characteristics is even smaller.
As discussed earlier, ternary or quaternary materials composed of III-V compounds have potentially desirable properties. Nevertheless, the ternary liquidus-solidus gap is generally quite large. (See M. B. Panish and M. Ilegems, "Phase Equilibria in Ternary III-V Systems", Vol. 7, Progr. Solid State Chem. (Ed. H. Reiss and J. McCaldin), N.Y. 1972 pp. 38-83). This fact seems to preclude the manufacture of useful bulk crystal devices from these materials.